---
title: Basic FastAPI Server
subtitle: "Build a simple Gmail agent with Composio and FastAPI"
image: "https://og.composio.dev/api/og?title=Basic%20FastAPI%20Server"   # image for socials
keywords: "fastapi, composio, openai, gmail, agent"
hide-nav-links: false
---

This cookbook will guide you through building agents equipped with tools using `Composio`, `OpenAI`, and `FastAPI`.

## Prerequisites

* Python 3.10+
* [UV](https://docs.astral.sh/uv/getting-started/installation/)
* Composio API key
* OpenAI API key

## Building an AI agent that can interact with `gmail` service

First, let's start with building a simple AI agent with Composio tools that lets the agent interact with `gmail`.

```python
from openai import OpenAI
from composio import Composio
from composio_openai import OpenAIProvider
import os

def run_gmail_agent(
    composio_client: Composio[OpenAIProvider],
    openai_client: OpenAI,
    user_id: str,  # Composio uses the User ID to store and access user-level authentication tokens.
    prompt: str,
):
    """
    Run the Gmail agent using composio and openai clients.
    """
    # Step 1: Fetch the necessary Gmail tools list with Composio
    tools = composio_client.tools.get(
        user_id=user_id,
        tools=[
            "GMAIL_FETCH_EMAILS",
            "GMAIL_SEND_EMAIL",
            "GMAIL_CREATE_EMAIL_DRAFT"
        ]
    )

    # Step 2: Use OpenAI to generate a response based on the prompt and available tools
    response = openai_client.chat.completions.create(
        model="gpt-4.1",
        tools=tools,
        messages=[{"role": "user", "content": prompt}],
    )

    # Step 3: Handle tool calls with Composio and return the result
    result = composio_client.provider.handle_tool_calls(response=response, user_id=user_id)
    return result
```

<Note>
This example demonstrates a basic agent without state management or agentic loops,
suitable for simple one-step tasks. For complex multi-step workflows requiring
memory and iterative reasoning, see our cookbooks with frameworks like LangChain,
CrewAI, or AutoGen.
</Note>

To invoke this agent, authenticate your users with Composio's managed authentication service.

## Authenticating users


To authenticate your users with Composio you need an authentication config for the given app. In this case you need one for gmail. 

To create an authentication config for `gmail` you need `client_id` and `client_secret` from your [Google OAuth Console](https://developers.google.com/identity/protocols/oauth2). Once you have the credentials, use the following piece of code to set up authentication for `gmail`.

```python
from composio import Composio
from composio_openai import OpenAIProvider

def create_auth_config(composio_client: Composio[OpenAIProvider]):
    """
    Create a auth config for the gmail toolkit.
    """
    client_id = os.getenv("GMAIL_CLIENT_ID")
    client_secret = os.getenv("GMAIL_CLIENT_SECRET")
    if not client_id or not client_secret:
        raise ValueError("GMAIL_CLIENT_ID and GMAIL_CLIENT_SECRET must be set")

    return composio_client.auth_configs.create(
        toolkit="GMAIL",
        options={
            "name": "default_gmail_auth_config",
            "type": "use_custom_auth",
            "auth_scheme": "OAUTH2",
            "credentials": {
                "client_id": client_id,
                "client_secret": client_secret,
            },
        },
    )
```

This will create a Gmail authentication config to authenticate your app’s users. Ideally, create one authentication object per project, so check for an existing auth config before creating a new one.

```python
def fetch_auth_config(composio_client: Composio[OpenAIProvider]):
    """
    Fetch the auth config for a given user id.
    """
    auth_configs = composio_client.auth_configs.list()
    for auth_config in auth_configs.items:
        if auth_config.toolkit == "GMAIL":
            return auth_config

    return None
```

<Note>
Composio platform provides composio managed authentication for some apps to
fast-track your development, `gmail` being one of them. You can use these
default auth configs for development, but for production, always use your
own oauth app configuration.
</Note>

Once you have authentication management in place, we can start with connecting your users to your `gmail` app. Let's implement a function to connect users to your `gmail` app via composio.

```python
from fastapi import FastAPI

# Function to initiate a connected account
def create_connection(composio_client: Composio[OpenAIProvider], user_id: str):
    """
    Create a connection for a given user id and auth config id.
    """
    # Fetch or create the auth config for the gmail toolkit
    auth_config = fetch_auth_config(composio_client=composio_client)
    if not auth_config:
        auth_config = create_auth_config(composio_client=composio_client)

    # Create a connection for the user
    return composio_client.connected_accounts.initiate(
        user_id=user_id,
        auth_config_id=auth_config.id,
    )

# Setup FastAPI
app = FastAPI()

# Connection initiation endpoint
@app.post("/connection/create")
def _create_connection(
    request: CreateConnectionRequest,
    composio_client: ComposioClient,
) -> dict:
    """
    Create a connection for a given user id.
    """
    # For demonstration, using a default user_id. Replace with real user logic in production.
    user_id = "default"

    # Create a new connection for the user
    connection_request = create_connection(composio_client=composio_client, user_id=user_id)
    return {
        "connection_id": connection_request.id,
        "redirect_url": connection_request.redirect_url,
    }
```

Now, you can make a request to this endpoint on your client app, and your user will get a URL which they can use to authenticate.

## Set Up FastAPI service

We will use [`FastApi`](https://fastapi.tiangolo.com/) to build an HTTP service that authenticates your users and lets them interact with your agent. This guide will provide best practices for using composio client in production environments.

### Setup dependencies

FastAPI allows [dependency injection](https://fastapi.tiangolo.com/reference/dependencies/) to simplify the usage of SDK clients that must be singletons. We recommend using composio SDK client as singleton.

```python
import os
import typing_extensions as te

from composio import Composio
from composio_openai import OpenAIProvider

from fastapi import Depends

_composio_client: Composio[OpenAIProvider] | None = None

def provide_composio_client():
    """
    Provide a Composio client.
    """
    global _composio_client
    if _composio_client is None:
        _composio_client = Composio(provider=OpenAIProvider())
    return _composio_client


ComposioClient = te.Annotated[Composio, Depends(provide_composio_client)]
"""
A Composio client dependency.
"""
```

Check [dependencies](./simple_gmail_agent/server/dependencies.py) module for more details.

### Invoke agent via FastAPI

When invoking an agent, make sure you validate the `user_id`.

```python
def check_connected_account_exists(
    composio_client: Composio[OpenAIProvider],
    user_id: str,
):
    """
    Check if a connected account exists for a given user id.
    """
    # Fetch all connected accounts for the user
    connected_accounts = composio_client.connected_accounts.list(
        user_ids=[user_id],
        toolkit_slugs=["GMAIL"],
    )

    # Check if there's an active connected account
    for account in connected_accounts.items:
        if account.status == "ACTIVE":
            return True

        # Ideally you should not have inactive accounts, but if you do, delete them.
        print(f"[warning] inactive account {account.id} found for user id: {user_id}")
    return False

def validate_user_id(user_id: str, composio_client: ComposioClient):
    """
    Validate the user id, if no connected account is found, create a new connection.
    """
    if check_connected_account_exists(composio_client=composio_client, user_id=user_id):
        return user_id

    raise HTTPException(
        status_code=404, detail={"error": "No connected account found for the user id"}
    )

# Endpoint: Run the Gmail agent for a given user id and prompt
@app.post("/agent")
def _run_gmail_agent(
    request: RunGmailAgentRequest,
    composio_client: ComposioClient,
    openai_client: OpenAIClient,  # OpenAI client will be injected as dependency
) -> List[ToolExecutionResponse]:
    """
    Run the Gmail agent for a given user id and prompt.
    """
    # For demonstration, using a default user_id. Replace with real user logic in production.
    user_id = "default"

    # Validate the user id before proceeding
    user_id = validate_user_id(user_id=user_id, composio_client=composio_client)

    # Run the Gmail agent using Composio and OpenAI
    result = run_gmail_agent(
        composio_client=composio_client,
        openai_client=openai_client,
        user_id=user_id,
        prompt=request.prompt,
    )
    return result
```

<Note>
Check [server](./simple_gmail_agent/server/) module for service implementation
</Note>

## Putting everything together

So far, we have created an agent with ability to interact with `gmail` using the `composio` SDK, functions to manage connected accounts for users and a FastAPI service. Now let's run the service.

<Note>
Before proceeding, check the [code](./simple_gmail_agent/server/api.py) for utility endpoints not discussed in the cookbook
</Note>


1. Clone the repository
   ```bash
   git clone git@github.com:composiohq/composio-fastapi
   cd composio-fastapi/
   ```
2. Setup environment
   ```bash
   cp .env.example .env
   ```

   Fill the api keys
   ```dotenv
   COMPOSIO_API_KEY=
   OPENAI_API_KEY=
   ```

   Create the virtual env
   ```bash
   make env
   source .venv/bin/activate
   ```
3. Run the HTTP server
   ```bash
   uvicorn simple_gmail_agent.server.api:create_app --factory
   ```
## Testing the API with curl

Assuming the server is running locally on `http://localhost:8000`.

### Check if a connection exists
```bash
curl -X POST http://localhost:8000/connection/exists
```

### Create a connection
Note: The body fields are required by the API schema, but are ignored internally in this example service.
```bash
curl -X POST http://localhost:8000/connection/create \
  -H "Content-Type: application/json" \
  -d '{
    "user_id": "default",
    "auth_config_id": "AUTH_CONFIG_ID_FOR_GMAIL_FROM_THE_COMPOSIO_DASHBOARD"
  }'
```
Response includes `connection_id` and `redirect_url`. Complete the OAuth flow at the `redirect_url`.

### Check connection status
Use the `connection_id` returned from the create step.
```bash
curl -X POST http://localhost:8000/connection/status \
  -H "Content-Type: application/json" \
  -d '{
    "user_id": "default",
    "connection_id": "CONNECTION_ID_FROM_CREATE_RESPONSE"
  }'
```

### Run the Gmail agent
Requires an active connected account for the `default` user.
```bash
curl -X POST http://localhost:8000/agent \
  -H "Content-Type: application/json" \
  -d '{
    "user_id": "default",
    "prompt": "Summarize my latest unread emails from the last 24 hours."
  }'
```

### Fetch emails (direct action)
```bash
curl -X POST http://localhost:8000/actions/fetch_emails \
  -H "Content-Type: application/json" \
  -d '{
    "user_id": "default",
    "limit": 5
  }'
```

These examples are intended solely for testing purposes.

## Using Composio for managed auth and tools

Composio reduces boilerplate for building AI agents that access and use various apps. In this cookbook, to build Gmail integration without Composio, you would have to write code to

- manage Gmail OAuth app
- manage user connections
- tools for your agents to interact with Gmail

Using Composio simplifies all of the above to a few lines of code as shown in the cookbook.

## Best practices

**🎯 Effective Prompts**:
- Be specific: "Send email to john@company.com about tomorrow's 2pm meeting" works better than "send email"
- Include context: "Reply to Sarah's email about the budget with our approval"
- Use natural language: The agent understands conversational requests

**🔒 User Management**:
- Use unique, consistent `user_id` values for each person
- Each user maintains their own Gmail connection
- User IDs can be email addresses, usernames, or any unique identifier


## Troubleshooting

**Connection Issues**:
- Ensure your `.env` file has valid `COMPOSIO_API_KEY` and `OPENAI_API_KEY`
- Check if the user has completed Gmail authorization.
- Verify the user_id matches exactly between requests

**API Errors**:
- Check the server logs for detailed error messages
- Ensure request payloads match the expected format
- Visit `/docs` endpoint for API schema validation

**Gmail API Limits**:
- Gmail has rate limits; the agent will handle these gracefully
- For high-volume usage, consider implementing request queuing
